1. Field of the Invention
The present invention, generally, relates to Microelectromechanical Systems (MEMs) processes commonly used in semiconductor manufacturing, but applied to composite materials and “smart materials” or “responsive materials”. More particularly, the present invention relates to methods for incorporating a negative thermal expansion system (NTEs) device in elastomer or soft composite materials and in conductive elastomer interconnects in microelectronic packaging.
2. Description of Related Art
In many areas of technology, the difference in coefficient of thermal expansion (TCE) between bonded parts or layers creates stresses that are highly problematic. In many cases such stresses are limiting factors because the strength of the materials or the interfaces between them are unable to withstand them during temperature excursion. When the materials in question have high elastic modulae, the TCE mismatch problem is exacerbated. When they are softer, the mismatch is partly mitigated by elastic deformation. However, this does not fully counter the problems associated with TCE mismatch, and indeed there is a class of uses for elastomers in technology for which the considerations are quite different. These are when the elastomer is employed to provide a restoring force while in compression. When such is the case, the restoring force will be reduced upon a decrease in ab-21solute temperature due to the TCE-driven contraction. Indeed, if the restoring force is small and the temperature decrease large, the elastomer can transition from being in compression to being in tension. This assumes an adhesive bond. If no such adhesive bond exists, or if the adhesion fails during the compression/tension transition, then contact may be lost altogether when the restoring force becomes less than zero. If the role of the elastomer is both to provide adequate restoring force and to provide conductivity (either electrical or thermal) then that conductivity will suddenly be interrupted upon loss of contact.
To minimize this problem materials have traditionally been engineered in a variety of ways to have a low TCE while balancing other necessary properties. One such approach has been to form composites with a low TCE material in a host polymer matrix. Typically, quartz (SiO2) filler in a thermoset polymer like epoxy. In another example, the organic fiber Kevlar is known to have a negative TCE in the fiber direction (only) and composites made with oriented Kevlar strands have reduced TCE in that direction. Many low or negative TCE materials have drawbacks, which have made them unattractive for some applications, notably microelecronics.
In addition, the control of thermal expansion is particularly important in elastomers (e.g. rubber), which has a notoriously high expansion coefficient limiting its use in many high technology applications. Of particular immediate interest is the fabrication of small conducting elastomer interconnect contacts for high-end microelectronic packaging. In traditional examples of such contacts, an electrically conducting material such as metallic silver particles are mixed into siloxane rubber and molded into small conducting contacts. These contacts are fabricated into a 2-dimensional array and used as a so-called Land Grid Array (LGA) connection between a chip module and a printed circuit board. However, because these contacts have a high TCE, they are unreliable and are rendered unsuitable for use in high performance computers where reliability on an individual contact basis must be measured in failure rates at the ppm to ppb level. This high reliability requirement stems from a full system dependence on non-redundant signal contacts—if even one out of many thousands fail, an entire node or the entire computer can fail. If the TCE could be reduced in such typical contacts while maintaining the desirable properties such as elasticity and conductivity, this would significantly increase the reliability. This in turn would reduce the cost of replacing chip modules in the field by allowing field replacibility of chip modules using LGA interconnects.
Herein we discuss an innovative approach based on the fabrication of a multitude of negative thermal expanding systems devices (NTEs) that have significantly negative coefficients of thermal expansion, and on the incorporation of such NTEs into an elastomer to form a composite with reduced, zero, or negative net TCE. This approach addresses a number of general engineering concerns such as the reduction of TCE-based stresses to levels that allows fabrication of structures not previously possible and such as extending the operating conditions under which elastomer composites will be able to maintain positive restoring forces to opposing surfaces. In particular we disclose herein the fabrication of LGA interconnect devices using such composite materials as the conducting elastomer.
These NTEs devices may also be used to form negative thermal expansion foams by fusing or adhering the NTEs together with no host elastomer.
The general concept of negative TCE micro machines is disclosed, as are process techniques and composite structures. Also disclosed are previously unidentified applications for negative TCE composites in general.